1. Field of the Invention
The present invention relates to a developing device used in a copying machine, a facsimile device, a printer and the like, and to an image forming apparatus using the developing device.
2. Description of the Related Art
There has conventionally been widely used an image forming apparatus with a developing device that uses a two-component developer consisting of toner and magnetic carrier. As this type of image forming apparatus, there is an image forming apparatus in which toner is replenished, according to need, from a toner container to a developer contained in a developing device that consumes toner as it conducts development, and thereby the toner density of the developer is maintained within a predetermined range. In such a configuration, because the carrier within the developer is repeatedly used without being consumed significantly, the coated layer on the surface layer of the carrier is worn when an image is output, or a toner resin or an additive is adhered to the coat layer. Consequently, the ability of the carrier to charge the toner decreases gradually, deteriorating the carrier. Toner charge amount decreases as the deterioration of the carrier progresses, causing scumming or toner scattering. Therefore, a serviceman is sent to the user of this type of image forming apparatus to replace the carrier regularly. For this reason, the maintenance cost and the cost per image formation increase.
Japanese Unexamined Patent Application No. 2005-292511 describes a developing device in which pre-mixed developer with a mixture of carrier and toner is replenished into a developer contained in the developing device to recover the toner density, and at the same time the increment of the developer is discharged from the developing device. In such a configuration, old carrier is discharged little by little from the developing device by discharging the developer, and at the same time new carrier within the pre-mixed developer is replenished to the developer contained in the developing device. Then, the carrier is replaced with new carrier little by little by performing the discharge and replenishment, whereby the carrier replacement work can be omitted.
Moreover, in this developing device, a developer discharge port for discharging the developer to the outside of the device is provided as developer discharge means at a predetermined height level of a supply conveyance path which conveys the developer in the axial direction of a developing roller while supplying the developer to the developing roller. In this developing device, when the pre-mixed developer is supplied and the amount of developer within the developing device increases, the developer bulk increases in the supply conveyance path. At this moment, the developer that has reached the height level of the developer discharge port in the position provided with the developer discharge port is discharged from the developer discharge port to the outside of the developing device.
However, in this developing device, the developer discharge port is provided in the middle of a developer supply region where the developer is supplied to the developing roller within the supply conveyance path. The developer conveyed within the supply conveyance path is scattered by its moving force or by the rotating force of a conveying member providing the developer with a conveying force when the conveying member is a conveying screw, and this developer is sometimes discharged from the developer discharge port. When the developer scatters and is discharged, the scattered developer might be discharged even if the developer is conveyed within the supply conveyance path in appropriate amount or less. In this situation, the developer is discharged, despite that the amount of developer within the developing device is not increased. If the developer is discharged from the developer discharge port despite that the amount of developer is equal to or lower than the appropriate amount, the amount of developer within the developing device might fall below the necessary amount, destabilizing the supply of the developer to a latent image carrier. If the supply of the developer to the latent image carrier is destabilized, image omission and other abnormal images occur.
Such problems are not limited to a developing device that uses a two-component developer, and thus might occur in any developing device that uses a one-component developer, as long as such a developing device is configured such that a developer is replenished by developer replenishing means and then the increment of the developer within the developing device is discharged by developer discharge means.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Application No. H05-127537, Japanese Unexamined Patent Application No. H06-258930, Japanese Unexamined Patent Application No. 2000-047474, and Japanese Unexamined Patent Application No. 2001-290369.